Spambot?

I have noticed recently that my computer sometimes runs very slow. Today I noticed that my cable modem is indicating activity even when IE is not open.

Does this indicate that my computer may be infected with one of the programmes that captures spare capacity and uses it to send out rogue e-mails?

About three weeks ago my AV software intercepted something and quaranteened it. symantec were not able to provide much information about it when i reported it. If it helps - it was filed at C:\windows\temp internet\files\content\IE5\KDEF05MR\counters.php